Surface sizing, i.e., the addition of sizing agents to the surface of a paper sheet that has been at least partially dried, is widely practiced in the paper industry, particularly for printing grades. Surface sizing leads to paper with improved water holdout (sizing), improved print quality, and increased toner adhesion. The most widely used surface sizing agent is starch, which is used to enhance the surface characteristics of the sheet, particularly for the purposes of controlling ink receptivity and porosity, and of increasing surface strength.
The growing use of ink jet printing, which generally uses aqueous-based inks, has placed more stringent requirements on paper properties, because the inks must provide printed characters with high optical density, minimal spread (also referred to as feathering or bleed) and sharp or clean edges (also referred to as wicking or edge roughness). The surface characteristics of the printing paper have been found to be a primary influence on these qualities of ink jet printing.
Polymer latexes are used for several functions in paper making processes. They are used as pigment binders for paper coatings, for paper saturants, as dispersing aids for other paper additives, and as sizing agents.
The paper coating process is completely different in function, composition and requirements when compared to surface sizing processes. Paper coating compositions have much higher viscosities than surface sizing compositions, and thus cannot be readily applied by a size press on a typical paper machine. Paper coatings contain pigment at 3 to 20 times higher than the level of polymeric binder; whereas in a typical surface size, pigments are optional, and if used, are present at levels less than the amount of polymeric binder.
The polymer latexes that have been used for sizing are cationic latexes. For example, U.S. Pat. No. 4,434,269 discloses sizing agents for paper that are copolymers of acrylonitrile or methacrylonitrile, C.sub.1 -C.sub.12 alkyl esters of acrylic acid and/or methacrylic acid. The copolymers are emulsified with a cationic polymeric emulsifier containing N,N'-dimethylaminoethyl acrylate or methacrylate, styrene, and acrylonitrile monomers.
U.S. Pat. No. 4,659,431 discloses sizing agents for paper that are copolymers of acrylonitrile or methacrylonitrile, styrene, and acrylates or methacrylates having 1 to 12 carbon atoms in the alcohol radical. The copolymers are emulsified with a cationic polymeric emulsifier containing monomers consisting of N,N'-dimethylaminoethyl acrylate or methacrylate, styrene, and acrylonitrile.
U.S. Pat. Nos. 5,116,924 and 5,169,886 disclose sizing agent dispersions containing cationic dispersant produced from the monomers: N,N'-dimethylaminoethyl acrylate and/or methacrylate; an acrylic and/or methacrylic acid ester of a C.sub.10 to C.sub.22 fatty alcohol; methyl acrylate and/or methacrylate; acrylic acid and/or methacrylic acid; and optionally butyl acrylate and/or butyl methacrylate and isobutyl acrylate and/or isobutyl methacrylate.